1. Field of the Invention
The present invention relates to a detectable device, and particular to a detectable device for attachment to movable items such as a magnetic tape in which data such as personal information is stored, other goods or the like in order to prevent unauthorized removal of a movable item, and further relates to a movable item detecting system comprising the detectable device and movable item detecting equipment, for detecting when a movable item is moved. In the specification, "detectable device" means a device in which radio waves transmitted from movable item detecting equipment is received and from which radio waves having a harmonic frequency are reradiated. The harmonic frequency is an integral multiple of the frequency of the received wave.
2. Description of the Invention
One example of the above-mentioned conventional detectable device is disclosed in Japanese Patent Laying-Open No. 226080 of 1987. FIG. 11 shows this detectable device.
The detectable device comprises a receiving antenna 1a, a diode 1d, and a transmitting antenna 1b. Receiving antenna 1a is connected through diode 1d to transmitting antenna 1b. DC biased coil 1c is connected in parallel with diode 1c. Radio waves of different frequencies Fa and Fb from each other sent from the movable item detecting equipment are received by the receiving antenna 1a of the detectable device, and supplied anode side of diode 1d . In the diode 1d, two frequencies Fa and Fb are converted to a frequency Fc being equal to either the difference between frequencies Fa and Fb or the sum of frequencies Fa and Fb. Radio waves having the frequency Fc are radiated from transmitting antenna 1b mounted on the cathode side of the diode 1d.
Another example of detectable device is disclosed in Japanese Patent Laying-Open No. 273475 of 1987. FIG. 12 shows this detectable device.
This detectable device comprises receiving antenna 2a and 2b, diode 2e, and transmitting antenna 2c and 2d. One end of the receiving antenna 2a and one end of the receiving antenna 2b are connected with the diode 2e on the anode side thereof, while the other end of receiving antenna 2a and 2b are connected with diode 2e on the cathode side thereof. One end of transmitting antenna 2c is connected with the anode side of diode 2e, while one end of the transmitting antenna 2d is connected with the cathode side of diode 2e. Radio waves sent from the transmitting equipment of the movable item detecting equipment are received by the receiving antenna 2a and 2b, its frequency converted to harmonic frequencies of the received radio waves, and radiated from the transmitting antennas 2c and 2d. Receiving antenna parts 2a and 2b form part of one folded dipole antenna. The total electric length of the receiving antenna 2a and 2b is equal to a half of the wave length of the received radio waves. The transmitting antenna 2c and 2d forms part of a dipole antenna of which total electric length is equal to a half of the wave length of one of the harmonic waves generated in the diode 2e, therefore the potential difference generated between both terminals of the diode 2e is high enough to generate the harmonic frequency components.
However, in the former example shown in FIG. 11, both radio waves with frequencies Fa and Fb sent from movable item detecting equipment are received by the receiving antenna 1a of the detectable device, and converted to frequency Fc being equal to either the difference between frequencies Fa and Fb or the sum of frequencies Fa and Fb. Therefore, a single antenna which can be operated with these three frequencies should be mounted on the detectable device, or three antennas which fit the respective frequencies should be mounted on the detectable device. Therefore, the structure is complicated and bulky. Further, miniaturization of the device is difficult. Furthermore, since a capacity between both terminals of the diode exists, current flowing through the diode becomes small at high frequencies so that the effective frequency conversion is quite difficult.
On the other hand, in the latter example shown in FIG. 12, the electric length of the receiving antenna 2a and 2b is equal to a half of the received wave length. At a comparatively high frequency of about 1 GHz used practically, the length of antenna reachs the order of 150 mm. Even if the antenna is folded, the size of the antenna reaches the order of 75 mm through 50 mm. Further, the transmitting antenna has a length of 75 mm. Therefore, the detectable device is too large to be attached to small-sized goods. To miniaturize the detectable device, the antenna must be shortened. However, the shorter the length of the antenna becomes, the lower the radiation resistance becomes. On the other hand, a diode has high impedance. Therefore, the impedance matching of the receiving antenna and the diode is impposible so that sufficient electric power cannot be sent to the transmitting antenna.
As above-mentioned, conventional detectable devices have problems that the smaller the detectable device is made, the lower the radiation resistance becomes. Therefore impedance matching of the receiving antenna and the diode is impossible, by which electric power applied to the diode is made smaller, which makes the effective reradiation of radio waves having the harmonic frequencies impossible.